1. Field of the Invention
The present invention relates to cleaning of a Chemical vapor Deposition (CVD) apparatus and more particularly, to an in-situ cleaning method of a reaction chamber of a CVD apparatus in which an elementary metal or metal compound film is formed on a semiconductor substrate or wafer through a chemical reaction of reducing decomposition of a metal halide gas.
2. Description of the Prior Art
In recent years, miniaturization of contact holes has been progressing more and more with the increasing integration level of Large-Scale Integrated circuits (LSIs) and consequently, the aspect ratio of the contact holes has been becoming larger and larger. Here, the aspect ratio is defined as a ratio of the depth of a contact hole with respect to the diameter thereof.
Under such the circumstance as above, the conventional wiring or interconnection film, which is typically made of aluminum (Al) deposited by a sputtering process, tends to have various disadvantages. For example, the contact resistance of the wiring or interconnection film becomes high and the wiring or interconnection film itself becomes discontinuous at the windows of the contact holes, which are due to the low step-coverage property of the film. Also, the Al film tends to be discontinuous due to the electromigration phenomenon during operation, thereby lowering its reliability.
To avoid these disadvantages, various metal plugs have been developed to electrically interconnect an upper conductive layer to a lower conductive layer. In this case, the upper and lower conductive layers are electrically connected together through the metal plugs that fill the contact holes of an intervening dielectric film between the upper and lower conductive layers.
A typical example of the metal plugs is tungsten (W) plugs formed by a plasma-enhanced CVD process with a good step coverage.
In the case of the W plugs, first, a thin titanium (Ti) film is formed on a dielectric film with contact holes by a sputtering process, so that the Ti film is deposited not only on the surface of the dielectric film but also in the contact holes thereof. The thin Ti film serves to lower the contact resistance with a silicon substrate as the lower conductive layer.
Next, a titanium nitride (TiN) film is formed by a sputtering process on the entire Ti film thus deposited. The TiN film serves to improve the adhesion strength of a W film to the Ti film and to prevent the W atoms in the W film from entering the silicon substrate. The Ti and TiN films serves as a metal barrier.
Subsequently, a W film for forming the W plugs is formed on the entire TiN film by a CVD process.
Finally, the unnecessary W, TiN, and Ti films on the surface of the dielectric film are etched back, thereby leaving selectively the W, TiN, and Ti films in the contact holes thereof. Thus, the W plugs located on the TiN and Ti films are formed in the respective contact holes of the dielectric film.
However, if the aspect ratio of the contact holes is further increased, the sputtered Ti and TiN films for the W plugs will become unable to have satisfactorily large thicknesses in the contact holes. This leads to such problems as increase in contact resistance and damage of the electronic devices or elements on the substrate.
To prevent these problems from occurring, the Ti and TiN films may be formed by CVD processes. In this case, however, the following problem tends to occur.
Specifically, if each of the Ti and TiN films is formed by a CVD process, the Ti or TiN films tends to be deposited not only on the substrate but also on the exposed inner surfaces of a reaction chamber of a CVD apparatus. The undesired Ti or TiN film that has been deposited on the inner surfaces of the chamber will be detached therefrom at the time when the Ti or TiN film has grown to have a specific thickness. The detached Ti or TiN film will become a cause of particulate contamination generated in the chamber. This problem will be explained in detail below with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B schematically show the typical configuration of a plasma-enhanced CVD apparatus.
A plasma-enhanced CVD apparatus 1100 shown in FIGS. 1A and 1B has a reaction chamber 1101, an upper electrode 1102 fixed onto the inner top wall of the chamber 1101, a susceptor or substrate holder 1103 fixed onto the inner bottom wall of the chamber 1101, a radio-frequency (RF) power supply 1109 provided outside the chamber 1101, a direct-current (DC) power supply 1104 provided outside the chamber 1101, and a vacuum pump system 1114 provided outside the chamber 1101.
The upper electrode 1102, which is electrically connected to the RF power supply 1109, has an inner space 1113 and emission holes 1102a. A specific RF power is supplied to the upper electrode 1102 on operation. The inner space 1113 communicates with gas sources (not shown) provided outside the chamber 1101 through a gas inlet 1106 of the reaction chamber 1101. The emission holes 1102a communicates with a reaction space 1112 of the chamber 1101. The supplied gasses to the inner space 1113 are mixed in the space 1113 and then, emitted through the emission holes 1102a to the reaction space 1112.
The inside of the reaction chamber 1101 (i.e. the reaction space 1112) communicates with the vacuum pump system 1114 through a gas outlet 1105 of the chamber 1101. A pressure-regulating valve 1108 is provided at the gas outlet 1105. The gas or gases existing in the reaction space 1112 is/are evacuated by the vacuum pump system 1114 to generate a vacuum atmosphere in the space 1112. The pressure in the space 1112 may be adjusted by the pressure-regulating valve 1108.
The susceptor 1103 is electrically connected to the DC power supply 1104. A specific DC voltage is applied to the susceptor 1103 and the substrate 1107 placed thereon on operation.
When a Ti film is formed on a silicon wafer or substrate 1107 on which a lot of semiconductor devices have been fabricated through the popular fabrication processes such as photolithography, dry etching, and film deposition, first, the silicon substrate 1107 is transported into the reaction chamber 1101 of the CVD apparatus 1100 and then, placed on the susceptor 1103. Prior to this step, a specific vacuum atmosphere has been generated in the reaction space 1112 of the chamber 1101.
Next, titanium tetrachloride (TiCl.sub.4), argon (Ar), and hydrogen (H.sub.2) gasses are supplied to the inner space 1113 of the upper electrode 1102 through the gas inlet 1106 of the reaction chamber 1101, and mixed therein. The mixture of TiCl.sub.4, Ar, and H.sub.2 gases thus produced is then emitted toward the substrate 1107 through the emission holes 1102a of the upper electrode 1102. Thus, the mixture of TiCl.sub.4, Ar, and H.sub.2 gases are introduced into the reaction space 1102 of the chamber 1101.
On the other hand, a specific DC bias voltage is applied to the substrate 1107 by the DC power supply 1104 and at the same time, a specific RF power is supplied to the upper electrode 102 by the RF power supply 1109, thereby making a plasma 1111 of TiCl.sub.4, Ar, and H.sub.2 in the reaction space 1102 of the chamber 1101, as shown in FIG. 1A.
Thus, a Ti film (not shown) with a thickness of approximately 5 to 30 nm is formed on the silicon substrate 1107 by a plasma-enhanced CVD process.
During this CVD process, undesired Ti films 1110 tend to be deposited on several areas of the inner walls of the vacuum chamber 1101, the upper electrode 1102, and the susceptor 1103, as shown in FIG. 1A. The undesired Ti films 1110 thus deposited will grow every time when the above CVD process is carried out.
If the undesired Ti films 1110 grows to have a specific thickness after the above CVD process is repeated approximately a hundred times in the plasma CVD apparatus shown in FIG. 1A, at least a part of the undesired Ti films 1110 tend to be detached from the chamber 1101, the upper electrode 1102, or the susceptor 1103. The detached Ti films 1110 will generate some particulate contamination in the reaction space 1112 of the chamber 1101.
Moreover, even if the undesired Ti films 1110 are not detached, the atmosphere in the reaction space 1102 is badly affected by the undesired Ti films 1110. Thus, the growth rate and/or the quality of the Ti film on the substrate 1107 will degrade.
Especially, if the particulate contamination is contacted with the substrate 1107 during the above CVD process for the Ti film, the wiring or interconnection lines tend to be in short-circuit. This problem lowers the fabrication yield of the semiconductor devices to be fabricated on the substrate 1107. Accordingly, the undesired Ti films 1110 deposited in the reaction chamber 1101 need to be removed by a cleaning process each time when the above CVD-process is repeated specific times.
An example of the conventional cleaning methods of this sort is shown in FIG. 1B, in which the undesired Ti films 1110 are removed by using a plasma 1121 of hexafluoroethane (C.sub.2 F.sub.6) and argon (Ar). The undesired Ti films 1110 are contacted with the plasma 1121 for approximately 30 minutes to be removed.
The plasma 1121 may be generated by using any other halide than C.sub.2 F.sub.6.
In this conventional cleaning method, to remove unwanted residual products generated through this cleaning process, a plasma of Ar and H.sub.2 is generated in the reaction chamber 1101 and is kept for approximately 10 minutes as a post-cleaning process. After this post-cleaning process is finished, an ordinary CVD process will be able to be restarted in this CVD apparatus 1100.
With the above-described conventional cleaning method, the reaction space 1112 of the chamber 1101 is sufficiently cleaned after the post-cleaning process. However, there is a problem that the cleaning gas such as C.sub.2 F.sub.6 and undesired residual products that have been produced during the post-cleaning process tend to be left in the supplying and evacuating tubes and/or valves such as the inlet port 1106, the output port 1105, and the pressure-regulating valve 1108, which adversely affects the atmosphere in the chamber 1101.
To avoid such the bad effect, a dummy CVD process needs to be conducted five times or more to thereby remove completely the undesired cleaning gas and residual products.
As another example of the previously-described metal plugs, titanium nitride (TiN) plugs formed by a thermal CVD process have been studied instead of the above-described W plugs. This is because the thermal CVD process has a better step-coverage property than the plasma-enhanced CVD process.
FIGS. 2A and 2B schematically show the typical configuration of a thermal CVD apparatus 1200 used for forming the TiN plugs. This apparatus 1200 has a same configuration as that of the plasma-enhanced CVD apparatus 1100 shown in FIG. 1A.
When a TiN film is formed on the silicon substrate 1107 on which a lot of semiconductor devices have been fabricated, first, the silicon substrate 1107 is transported into the reaction chamber 1101 of the CVD apparatus 1200 and then, placed on the susceptor 1103. Prior to this step, a specific vacuum atmosphere has been generated in the reaction space 1112 of the chamber 1101.
Next, titanium tetrachloride (TiCl.sub.4), ammonia (NH.sub.3), and hydrogen (H.sub.2) gasses are supplied to the inner space 1113 of the upper electrode 1102 through the gas inlet 1106 of the reaction chamber 1101, and mixed therein. The mixture of TiCl.sub.4, NH.sub.3, and H.sub.2 gases thus produced is then emitted toward the substrate 1107 through the emission holes 1102a of the upper electrode 1102. Thus, the mixture of TiCl.sub.4, NH.sub.3, and H.sub.2 gases are introduced into the reaction space 1102 of the chamber 1101.
Since no plasma is used, no DC bias voltage is applied to the substrate 1107 and no specific RF power is supplied to the upper electrode 102, as shown in FIG. 2A.
Thus, a TiN film (not shown) is formed on the silicon substrate 1107 by a thermal CVD process.
During this CVD process, undesired TiN films 1210 tend to be deposited on several areas of the inner walls of the vacuum chamber 1101, the upper electrode 1102, and the susceptor 1103. as shown in FIG. 2A. The undesired TiN films 1210 thus deposited will grow every time when the above CVD process is carried out.
In the case of the TiN plugs, it is required that a TiN film is formed to have a large thickness of several thousands angstroms through a single CVD process. Therefore, the undesired TiN films 1210 tend to have a larger thickness than that of the Ti films 1110. Accordingly, the interval of the chamber cleaning becomes shorter than the case of the Ti film.
For example, if the above-described chamber cleaning method using the plasma 1121 of C.sub.2 H.sub.6 and Ar is applied, as shown in FIG. 2B, this cleaning method needs to be conducted each time when the CVD process is repeated approximately 50 times which is less than approximately 100 times for the Ti films 1110.
Further, unlike the case of the Ti film, a thermal reaction is used for deposition of the TiN film. Therefore, the undesired TiN films 1210 tend to be deposited on the wider areas of the inner exposed surfaces of the reaction chamber 1101 than the case of the Ti film, as shown in FIG. 2B.
Thus, even if the above-identified chamber cleaning method shown in FIG. 2B is applied to the TiN films 1210, there is a problem that a part of the undesired TiN films 1210 tends to be left, as shown in FIG. 2C. This is because the plasma 1211 is locally generated at the area between the upper electrode 1102 and the susceptor 1103 and because the anisotropically etching action of the plasma 1211 is blinded by the upper electrode 1102 and the susceptor 1103.
There is another problem that the thermal CVD apparatus 1200 needs to be equipped with a very-expensive plasma generator for the purpose of chamber cleaning alone. This raises the fabrication cost of the thermal CVD apparatus 1200 and that of the semiconductor devices fabricated with the use of this apparatus 1200.